1. Field of the Invention
This invention relates to the field of wafer defect inspection, and in particular to wafer inspection using charged particle beams.
2. Description of the Related Art
Defect inspection of semiconductor wafers and masks for IC manufacturing is an accepted production process for yield enhancement. The information obtained from a wafer defect inspection tool can be used to flag defective dies for repair, or to improve wafer processing parameters. The systems used for inspection are typically optical in nature and such systems are limited in resolution due to the wavelength of the measuring light (200-400 nm). Although possible, it becomes increasingly difficult to detect defects that are smaller than the wavelength of the measuring light. Defects can range in both size and shape, and have been measured to be as small as one quarter the size of the critical dimension (CD) (Hendricks et. al., SPIE vol. 2439 pp. 175-183, 1995). For example, in order to short two adjacent traces, a conductive shorting defect can be significantly thinner than the traces, and yet still cause the circuit to fail. As the feature sizes on semiconductor wafers drops below 0.18 μm, conventional optical inspection techniques will have difficulty detecting the smaller-sized defects.
To overcome this resolution limit, several electron beam inspection systems have been designed. The formation or creation of an image in an electron beam inspection system is similar to that of a Scanning Electron Microscope (SEM). A primary beam of electrons from an electron column is focused to a small spot on an object that is to be imaged (e.g. a silicon wafer containing microcircuits). The primary beam of electrons interacts with the object such that the object emits electrons. The electrons emitted from the object are typically classified into two groups: those with low energy (less than roughly 50 eV) are called secondary electrons; those that are emitted with high energy (close to the energy of the primary beam) are called backscattered electrons. Secondary electrons are emitted from a region close to the surface of the object and therefore give topographical information. Backscattered electrons can come from deeper within the object and typically give information about the atomic composition of the object, with only minor topographical information. Signal information is collected to form an image by detecting either secondary, backscattered or both types of electrons. An image is formed by scanning the primary beam over the object in a raster fashion—such as in a television or cathode ray tube—and collecting the secondary/backscattered electrons. The signal from the detected electrons is displayed synchronously with the primary beam raster on a display monitor, or stored on photographic film or computer memory. Magnification is achieved by the ratio of the size of the raster scan on the object and the size of the display. The information from images can be divided into small regions called pixels for digitization and computer manipulation. Typically the size of the focused electron beam is between one and two times the pixel size for optimum image quality.
In order to optimize the imaging process using an e-beam system, a knowledge of the materials is required. The object materials for imaging under an electron beam fall into two categories: conductive and insulating materials. Special care must be used when imaging insulators as they charge under electron bombardment. In some cases charging of the insulators can be so severe that no useful image can be obtained. The charging of materials occurs when there is an imbalance between the number of electrons striking the object and the number of electrons leaving the object. The number of electrons that leave the surface is dependent on the primary beam energy. The ratio of the number of electrons leaving the surface to the number of electrons hitting the surface is typically larger than one when the primary beam is in the energy range of 200-2000 eV. For insulating materials when the number of electrons leaving the surface is greater than the number entering the surface, the surface charges positively until an equilibrium voltage is reached. When the primary beam energy is outside this range the number of electrons leaving the surface is less than the number entering and the surface charges negatively until an equilibrium voltage is reached. Therefore it is practical to image insulating materials in low voltage mode with the primary beam energy less than 2000 eV. Integrated circuits or microcircuits are comprised of both conducting and insulating materials and therefore low beam voltage operation is preferred.
Electron beam systems can have much higher resolution than optical systems because the wavelength of the electron can be in the angstrom regime. Present SEMs can have resolution down to 10 Åor even less, operating at currents less than 1 nA. Electron beam systems are limited in the speed at which they can inspect wafers. In present systems, throughputs of approximately 30 minutes per square cm have been reported (Hendricks et al, SPIE vol. 2439, pg. 174). Thus to inspect an entire 300 mm diameter silicon wafer, approximately 70 hours will be required. These systems can be used in a sampling mode where only several dies are inspected, thereby increasing throughput to several hours per wafer. These systems have been effective for research and product development, but are impractical for volume production. These electron beam inspection systems have a single electron beam that images the wafer. There are two insurmountable problems for a single electron column system to achieve sufficient wafer inspection throughput. First, space charge effects significantly reduce resolution when the electron current is increased for adequate signal-to-noise ratios. Second, the data rate from a single column is obtained serially from the electron detectors within the column; as imaging resolution increases, this data rate cannot be handled by present-day image computers. An estimated 10-20 Gbytes/s data rate is required.
An electron beam inspection system with high resolution and throughputs of several wafers per hour could be used to inspect parts of each wafer, or perhaps to inspect one or more complete wafers out of a cassette during the time required to complete a single processing step for all the wafers. Such an in-line wafer defect inspection tool would significantly increase the yield of ICs for chip manufacturers.